In the acoustic arts, it is conventional to place a sound adsorber material in a back volume of a loudspeaker device to acoustically enlarge the back volume in a virtual sense. In a loudspeaker device having a physically small back volume, a sound adsorber material lowers the resonance frequency of the loudspeaker device to a value that is similar to a loudspeaker device with a physically larger back volume.
More specifically, sound adsorber materials disposed in the back volume of a loudspeaker device improve its sound characteristics, e.g., the wideband performance, and the apparent acoustic volume of the loudspeaker. Examples of sound adsorber materials include zeolite materials, zeolite-based materials, silica (SiO2), alumina (Al2O3), zirconia (ZrO3), magnesia (MgO), tri-iron tetroxide (Fe3O4), molecular sieves, fullerene, carbon nanotubes, and activated carbon or charcoal. Zeolite materials and zeolite-based materials are electrically isolating, unlike activated carbon. Since zeolite materials and zeolite-based materials are electrically non-conductive, they do not affect the electrical components (e.g., the antenna, the battery, the internal electronics, etc.) of a device that incorporates a loudspeaker device having such a sound adsorber material. In addition, the non-conductive zeolite material or zeolite-based material will not cause short circuits if it becomes loose within the device. Furthermore, the packaging of zeolite materials and zeolite-based materials is much easier than in case of activated carbon woven fabrics.
A problem may arise in the insertion or placement of sound adsorber materials consisting of or at least comprising powder, loose particles, or loose grains in the back volume of the loudspeaker device. Furthermore, the back volume of a miniature loudspeaker, such as a loudspeaker device placed in mobile phones, headsets, etc., is often constrained by other circuit components in the immediate physical area surrounding the loudspeaker, and sometimes the shape of the back volume is complex and not acoustically desirable. A conventional technique uses tubes that encase a sound adsorber material, but these usually do not fit well into a back volume having a complex shape. A direct insertion of the sound adsorber materials into the back volume can be practically difficult. Furthermore, if not securely packaged, the sound adsorber materials can enter the different components of the loudspeaker device, as well as the handheld device that uses the loudspeaker device, and can therefore damage the loudspeaker device, or the handheld device that includes the loudspeaker device as a component
U.S. application Ser. No. 13/818,374, which is incorporated by reference in its entirety into this disclosure, discloses an audio system that comprises an electro-acoustic transducer or loudspeaker with a housing that forms a resonance volume to improve the quality of the emitted sound. The audio system disclosed in application Ser. No. 13/818,374 comprises a zeolite particulate material or a substantially ball-shaped zeolite granulate material that fills a portion of the resonance volume of a loudspeaker. Zeolite material is a sound adsorbing material that, depending on its formulation, results in a virtual acoustic enlargement of the volume of the resonance space by a factor of 1.5 or greater. As a result, the volume of the housing of the speaker that contains the zeolite material can be made smaller compared to a housing of a speaker filled with air.
The packaging of a zeolite-based material for use as a sound adsorber inside the back volume of a miniature loudspeaker, such as the type usually found in today's handheld consumer electronic devices, has been challenging. The zeolite materials disclosed in the application Ser. No. 13/818,374, although not electrically damaging, can interfere with the proper operation of a miniature loudspeaker, and potentially other components within a handheld consumer electronics device, if not properly contained within the device. In addition, due to the typically limited space within the back volume portion of a miniature loudspeaker, efficient gas exchange can be impeded and the efficiency of the zeolite-based sound adsorber can be lessened by design restrictions. Although the back volume of the miniature loudspeaker might be completely filled with a zeolite-based sound adsorber, if only a limited amount of sound adsorber surface area is exposed to pressure changes caused by acoustic transducer movement, the resonance frequency shift disclosed in application Ser. No. 13/818,374 is limited.